Need advice. I'm searching ideas and stats for a BECM monster for my campaign.

The characters are a party of high-level adventurers going through a (heavily modified) CM1: Test of the Warlords. No clerics: we have a lvl19 fighter, a lvl 19fighter/thief, two wizards (lvls 17 & 12).

The concept for the creature is: a powerful alchemical/magical/demonic beast/thing, sent millennia ago from the Alphatian empire trying to conquer the Norwold provinces as a war-beast and exterminator.
The thing then went wild and hid into dark caves near a remote enchanted vale/hamlet (think of Shangri-la myths).
Powerful shamanic priests of a Sun-god cult banished the creature into the dephts of the caves and warded the entrance with powerful spells. The PCs obviously broke the beast free and now it will be their business to put an end to its reign of terror.

Now think of a Lovecraftian feel to the creature and situation and add some science-fantasy feel. The beast could also be partly constructed or alchemical in nature.

Would like the creature to have:
- Immunity to low and mid level magical spells
- Immunity or high reistande to mind affecting effects, poisons, elements and mundane hazards
- Be hit by +3 or better weapons
- Possess some nasty spell-like power
- Possess powerful physical attacks and maybe an area attack (breath weapon?)
- High AC and high HP total

Suggestions, anyone?
Thank you in advance!
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